backends/drm: Provide a way to force color management
requested to merge work/zzag/backend-drm-allow-to-enable-color-management-on-machines-without-hdr into master
It can be handy if kwin runs on a setup not capable of HDR or without an ICC profile. The output management protocol has not been touched because the long term goal is to have color management enabled by default. This is rather for testing purposes.